Ultrasound testing is a non-destructive, acoustic test method for testing test objects composed of materials which conduct sound, such as metals. Ultrasound testing is attractive for many applications, particularly because of the capability to also detect internal defects which are located under the surface of the test object. In conventional ultrasound-based test systems, the ultrasound waves which are used for testing are produced by a piezoelectric transducer, and are introduced into the test object via a coupling medium, for example, water.
Electromagnetic acoustic transducers (EMATs) are an alternative to this conventional ultrasound technology, and can be used for testing all sufficiently electrically conductive materials. An electromagnetic acoustic transducer, which is also referred to for short in the following text as an EMAT, is used to produce ultrasound waves directly in the material of the test object, based on the electromagnetic principle. For this purpose, an EMAT has a magnet system for production of a magnetic field, which is intended to penetrate into the test object, and an inductive coil arrangement for production of an electromagnetic alternating field, which is superimposed on this magnetic field in the test object, and for detection of electromagnetic alternating fields emitted from the test object. The coil arrangement which has been excited by high-frequency AC voltage induces eddy currents in the area close to the surface of the test object. The charge carriers which are moved in this case are in this case moved in the magnetic field which is produced in the test object by the magnet system. The Lorentz force which is produced acts as a periodic force on the solid body structure of the material of the test object and in this way produces ultrasound waves directly within the test object, which ultrasound waves propagate in the material of the test object and can be used for testing. Therefore, in principle, ultrasound testing by an EMAT does not require any coupling medium, for example, water, thus allowing EMAT technology to also be used, for example, for testing metals in the hot state. Ultrasound testing by EMAT can be used both for fault testing in metals and metal alloys and for determination of geometric parameters, such as wall thickness, diameter and the like, of test objects composed of electrically conductive material.
DE 11 2005 000 106 T5 (which corresponds to WO 2005/083419) describes a conventional electromagnetic acoustic transducer which has a housing with a working surface facing the test object. A magnet system having permanent magnets for production of a continuous magnetic field, as well as an inductive coil arrangement, in the vicinity of the working surface, for electromagnetic production of scanning pulses and for reception of pulses emitted from the test object are arranged in the housing. The magnet system has at least three permanent magnets, which have a rectangular cross section and are arranged immediately adjacent to, and parallel to, the working surface of the EMAT in the housing. In this case, the central magnet has vertical polarization with respect to the working surface, while each of the adjacent magnets at the side have horizontal polarization. Magnetic flux concentrators are fitted between the permanent magnets of the magnet system and the coil arrangement such that the magnetic flux originating from the central magnet is in each case concentrated onto an induction coil which is fitted under a concentrator. The EMAT formed in this way is intended to have high sensitivity and a fairly broad working range, and to be physically compact at the same time.
WO 2007/013836 A1 discloses an electromagnetic acoustic transducer having an outer housing and an inner housing, which is fitted within the outer housing. The magnet system, with a plurality of permanent magnets, is accommodated in the inner housing. The inductive coil arrangement and the associated magnetic field concentrators are fitted to a mounting plate in the outer housing such that, when the inner housing is in a working position, the permanent magnetic field produced by the magnet system is concentrated on the coil arrangement. The inner housing is fitted such that it can move within the outer housing such that it can be moved away from the working position (magnetic flux contact with the coil arrangement) by linear movement or rotation with the aid of a drive which is fitted to the outer housing, this allowing the EMAT to be taken out of use. The aim of this is to make it possible to quickly lift the transducer off a test object without this being impeded by the magnetic field produced by the magnet system.
While testing, the inductive coil arrangement should be located as close as possible to the surface of the test object to achieve a high test sensitivity. To protect the coil arrangement in the case of known electromagnetic acoustic transducers, a protection device composed of wear-resistant material, for example, in the form of a ceramic protection plate, is located in front of the coil arrangement, that is to say between the coil arrangement and the surface of the test object. To avoid increasing the distance between the coils of the coil arrangement and the test object any more than necessary, the material of the protection layer should have only a small thickness. As a further constraint, the material should not, be significantly electrically conductive, because of the principle of the method for production of the ultrasound.
In general, electromagnetic acoustic transducers are used within a test apparatus to carry out the test process in an automated manner. In this case, an EMAT is passed over the surface of a test object to be examined such that the entire area of the test object to be examined can be covered. In industrial use, it is desirable for the test time to be as short as possible in this case to maximize the throughput of tested material. Relatively high relative speeds between the EMAT and the material of the test object are therefore desirable.
When testing ferromagnetic materials, relatively strong magnetic attraction forces can occur between the test objects and an EMAT that is being passed along the surface of the test object, and this can lead to considerable wear to the protection elements when there is a sliding contact between the EMAT and the surface of the test object, particularly when the relative speeds are relatively high.
As an alternative, the surface of the test object can be scanned without any touching contact by having a small air gap between the surface of the test object and the EMAT, which air gap should be at least sufficiently large that collisions do not occur, or occur only rarely, between the EMAT and the material of the test object, for example, because of irregularities on the surface of the test object. However, any increase in the distance between the surface of the test object and the coil arrangement results in a drastic decrease in the test sensitivity.
It could therefore be helpful to provide an electromagnetic acoustic transducer for ultrasound testing which allows test objects to be scanned at a high relative speed, but without significant restrictions in terms of the life of the EMAT or the test sensitivity.